Intraspecific somatic cell hybrids between cultured Chinese hamster (CH) cell mutants, with mutations in a series of linked genes, will be used to construct a relatively fine structure genetic map of four and possibly 5 genes linked to CH chromosome 2. We will also use these hybrids to examine the mechanisms involved in gene segregation and to determine through direct genetic analysis what role mitotic recombination plays in segregation. Interspecific hybrids between the same CH mutants and normal human cells will be used to construct a genetic map of and order the corresponding human genes on human chromosome 5, which will provide significant information concerning the evolution of this human chromosome. Interspecific hybrids between normal cells and a mutant CH cell line with a thermolabile asparagyl-tRNA synthetase (asnRS) will be used to assign the gene encoding this enzyme to a specific human chromosome, in order to begin expanding our now meager knowledge of the genetic map of genes encoding components of the protein synthetic machinery. This gene is also of particular interest because it appears to be especially susceptible to mutagenesis and is very amenable to classic genetic analysis. Using a combination of somatic cell genetic and recombinant DNA techniques, we propose to clone the gene encoding asnRS. This will ultimately enable us to examine directly the location and nature of specific nucleic acid changes in many of the over 100 mutants we have isolated with alterations in this gene. These experiments can provide a wealth of information relating gene organization and gene structure to gene expression and especially gene mutation.